1. Field of the Invention
This invention relates to an engaging member, for a surface fastener, in which a plate-like substrate and a multiplicity of hooks are molded of thermoplastic resin by extrusion or injection molding, and more particularly to a hook structure which has both flexibility and toughness like a monofilament though molded and is very durable.
2. Description of the Related Art
A hooked surface fastener has been known long since in which hooks are formed by cutting loops of monofilaments woven into a woven cloth. With this type of surface fastener, flexibility of the woven cloth and flexibility of the monofilaments combine to make a very smooth touch when the hooks come into and out of engagement with loops of the companion fastener member. Additionally, since the monofilaments forming hooks are treated with drawing process, they are excellent in toughness against pulling and bending though small in cross-sectional area. Further, since the hooks can be formed in high density depending on the structure of the woven cloth, this type of surface fastener has a high engaging rate and hence can survive in repeated use. However, with this woven-cloth-type surface fastener, since the hooks tend to deform as they give a very smooth touch during engaging and peeling, there is a limit in engaging strength. Further, partly since the quantity of material is large and partly since a large number of process steps are required, it is difficult to reduce the cost of production.
To this end, an alternative molded surface fastener has been developed in which the substrate and the hooks are simultaneously and integrally molded by extrusion molding. The molding technology for this type surface fastener is disclosed in, for example, U.S. Pat. No. 3,312,583 and WO 87/06522. In this molding method, a number of mold discs, each of which has a number of hook-forming cavities in its peripheral side surfaces, and a number of spacer discs, each of which has opposite flat side surfaces, are fixedly arranged alternately in a laminate drum. Hooks, which have been molded in cavities in the peripheral surface of the rotating drum integrally with a plate-like substrate, are removed, together with the substrate, from the peripheral surface of the drum. The reason why the spacer discs must be used in the prior art is that the whole contour of the individual hooks could not have been made in a single mold.
However, with the prior art integrally molded surface fastener, because of technological difficulty in molding process, it is impossible to obtain a delicate shape like the woven cloth type. Since the orientation of molecules of the molded hooks is poor, the degree of toughness of the hooks is very low if the hooks have the same size as those of monofilaments, thus making the surface fastener not suitable for practical use. Further, in the hook structure, the cross-sectional shape of its rising portion is simple so that the hook tend to fall flat from the base of the rising portion. As a result, the hooks would not restore its original posture after repeated use so that its engaging rate with loops of the companion fastener member would be lowered. Consequently, the size of the individual hooks must be increased in order to secure a desired degree of toughness. And the number of hooks per unit area (hook density) would be reduced so that the engaging rate of the hooks with companion loops will be lowered.
In order to eliminate the foregoing problems, an alternative hook structure which enables a smooth touch like the woven-type surface fastener, a high engaging rate and secures adequate toughness and durability in repeated use has been proposed by, for example, Japanese Utility Model Laid-Open Publication No. HEI 4-31512 (U.S. Pat. No. 5,131,119). In the molded surface fastener, each hook comprises a rising portion, which has a rear surface rising obliquely in a smooth curve from the substrate and a front surface rising upwardly, and a hook-shape engaging portion extending forwardly from the upper end of the rising portion. And the hook has a varying cross-sectional area increasing progressively from the distal end of the hook-shape engaging portion toward the base of the rising portion. Further, the rising portion has reinforcing ribs projecting from its respective side surfaces. This reinforcing ribs serve to keep the rising portion free from falling flat and also enable the rising portion and the hook-shape engaging portion to be reduced to minimum thicknesses which are enough to survive against a stress due to a required engaging strength.
The present inventors made a further study on the reinforcing ribs and found that the shape and arrangement of the reinforcing ribs gave a considerable influence on the distribution of stress of the hooks when the surface fastener is peeled. Thus as the shape and arrangement of the reinforcing ribs are changed, the distribution of their internal stress also will vary so that stresses tend to concentrate locally in the hook due to the compression and expansion.
The majority of conventional hooks which have been put into practice since the present inventors developed it has a structure shown in FIG. 5(b) of the accompanying drawings. As shown in FIG. 5(b), the hook 10' is composed of a rising portion 11' which has a rear surface 11a' rising obliquely in a smooth curve from the surface of a plate-like substrate 14' and a front surface 11b' rising upwardly, and a hook-shape engaging portion 12' extending forwardly from the upper end of the rising portion 11' and curving downwardly. The hook 10' has a varying cross-sectional area increasing progressively from the distal end of the hook-shape engaging portion 12' toward the base of the rising portion 11'. The rising portion 11' has on each of opposite side surfaces a reinforcing rib 13' having an arcuate upper surface and front and rear surfaces outwardly curving toward the base. The reinforcing rib 13' has a height about 2/3 of a vertical line segment between the surface of the plate-like substrate 14' and a peak O.sub.1 ' of the lower surface of the hook-shape engaging portion 12' with the distal end curving downwardly. The reinforcing rib is located in a position toward the front surface of the hook 10'.
However, in the case where the shape and arrangement of the reinforcing ribs are as mentioned above, a large stress concentration due to the expansion and compression would occur at the hook side and rear-surface side of the hook-shape engaging portion 12' above the peak of the reinforcing rib 13', as shown in FIG. 5(A). Accordingly, when load is exerted repeatedly on the hook-shape engaging portion 12', the hook-shape engaging portion 12' will become fragile around the peak of the reinforcing rib 13'.